1. Field of the Invention
The present invention relates to steel with excellent delayed fracture resistance, its shaped articles, and methods of production of the same, more particularly relates to steel with excellent delayed fracture resistance suitable for the production of high strength bolts of a tensile strength of 1600 MPa or higher, its shaped articles, and methods of production of the same.
2. Description of the Related Art
The improvement of performance and reduction of weight of automobiles and industrial machinery and the increasingly larger size of building structures have led to demands for development of steel for higher strength bolts.
The types of steel being currently generally used for high strength bolts are the JIS standard SCM435, SCR435, and other low alloy steel produced accompanied by quenching and tempering. However, these types of steel decrease in delayed fracture resistance when the tensile strength exceeds 1200 MPa, so the risk of breakage due to delayed fracture during use of the bolts increases. Therefore, increase of the strength to this level and higher has been impossible in practice.
Steel for high strength bolts designed to improve the delayed fracture resistance are being proposed by various companies. For example, Japanese Patent Publication (A) No. 5-148576 and Japanese Patent Publication (A) No. 5-148580 describe to prevent the light carburization and grain boundary oxidation occurring during heat treatment of bolts and to increase the amount of Mo to improve the tempering softening resistance so as to improve the delayed fracture resistance.
Further, for example Japanese Patent No. 2739713 describes an invention complexly adding Mo and V to prevent the formation of Fe3C and thereby improve the delayed fracture resistance of 1400 MPa class bolts.
Further, for example, Japanese Patent Publication (A) No. 2001-32044 describes an invention adding a relatively large amount of Mo and performing high temperature tempering so as to improve the delayed fracture resistance of 1500 MPa class and higher bolts. This is art effective for improving the delayed fracture resistance of bolts with tensile strengths of about 1500 to 1700 MPa.